Thanks for the reply Richard!

The best would of course be if the player himself could train the unit at any time, but after that not be able to train any more of the same unit (until it's killed). The purpose I was wondering is because I'm working on the warhammer mod, and we are trying to find a way of preventing people from training armies of greater daemons (which would be very "un-fluffy", and which is not possible in the table top game).

The thing with giving the player the daemon at the start of the campaign is that it would most likely get killed quite soon and then be gone forever (unless there's for example a way of modding the probability that the unit heals after a battle, but that has more to do with the way it was "taken down" than which unit it actually is, hasn't it?).